1. Field of the Invention
The present invention relates to a video signal encoding apparatus, and, more particularly, to a video signal encoding apparatus for converting a video signal into video data by encoding it in a high efficiency manner.
2. Description of the Related Art
There has been disclosed in, for example, Japanese Patent Laid-Open No. 63-1183, a video signal transmission system adapted to a TV telephone system, a conference telephone system and the like in which video signals each of which is composed of moving pictures are encoded in a high efficiency manner by using data encoded in an intraframe manner and data encoded interframe manner so as to be converted into difference data. As a result, the quantity of data of the video signals to be transmitted can be compressed so as to be transmitted through a passage which involves a limit of the quantity of data to be transmitted.
For example, as shown in FIG. 1, when the video signal transmission system of the type described above transmits images PC1, PC2, PC3, . . . , which constitute a moving picture, at points t=t.sub.1, t.sub.2, t.sub.3, . . . , it improves the transmission efficiency by subjecting the video data to be transmitted to a compression process by utilizing the characteristics of the video signal such that the auto-correlation is enlarged with the lapse of time. The intraframe encoding process is performed in such a manner that the images PC1, PC2, PC3, . . . , are subjected to a compression process in which, for example, pixel data is subjected to a comparison with a predetermined reference value so as to obtain the difference. As a result, the data of each of the images PC1, PC2, PC3, . . . , is transmitted by a data quantity which has been compressed by utilizing the cross-correlation between image data items in the same frame.
On the other hand, as shown in FIG. 1, the interframe encoding process is performed in such a manner that image data items PC12, PC23, . . . , each of which is the difference between image data items of the adjacent image PC1 and PC2, PC2 and PC3, . . . , are obtained. Furthermore, the thus obtained image data items PC12, PC23, . . . , are transmitted together with an image obtained by subjecting the initial image PC1 at the time t=t.sub.1 to the intraframe encoding process.
As a result, the images PC1, PC2, PC3, . . . , can be transmitted in the form of digital data the quantity of which has been significantly reduced in comparison to a case where all of image data items of the images PC1, PC2, PC3, . . . , are transmitted since they have been subjected to a high efficiency encoding process.
The above-described process of encoding the video signals can be applied to a case where video signal data obtainable from the encoding process performed in a video signal encoding apparatus 2 is transmitted to a video signal decoding device 4 via a transmission passage 3 in a video signal transmission system 1 structured as shown in FIG. 2.
The video signal encoding apparatus 2 subjects an input video signal VD.sub.IN to a previous process in its analog/digital conversion processing portion 11. Then, the video signal encoding apparatus 2 converts a brightness signal and a chromatic signal which constitute the input video signal VD.sub.IN into transmission unit block data S11 of a predetermined number of pixels (which are constituted by, for example, 8 pixels in the horizontal direction.times.8 pixels in the vertical direction) so as to transmit it to a video data encoding circuit 12.
The video data encoding circuit 12 receives predicted present frame data S12 formed by a prediction encoding circuit 13 so as to obtain difference data S13 from the transmission unit block data S11 so that encoding data between frames is generated (hereinafter called an "interframe encoding mode"). As an alternative to this, the video data encoding circuit 12 receives, as the predicted present frame data S12, reference value data and obtains data S13' from the transmission unit block data S11 so that intraframe encoding data is formed (hereinafter called an "intraframe encoding mode"). As a result, the difference data S13 obtainable in the image data encoding circuit 12 by the interframe encoding process or data S13' by the intraframe encoding process is supplied to a discrete cosine transformation (DCT) circuit 14.
The discrete cosine transformation circuit 14 orthogonally transforms the difference data S13 so as to supply transformation encoding data S14 to a quantizing circuit 15. As a result, the quantizing circuit 15 transmits quantized video data S15.
The quantized video data S15 obtained in the quantizing circuit 15 is again compressed in a variable length encoding circuit (VLC circuit) 16 before it is supplied to a transmission buffer memory 17 as transmission video data S16.
The transmission buffer memory 17 temporarily receives the transmission video data S16 supplied from the variable length encoding circuit 16 before the transmission buffer memory 17 transmits it as transmission data S17 to the transmission passage 3 via an interface circuit 18 at a predetermined data transmitting speed so that the transmission data S17 is transmitted to the video signal decoding device 4.
Furthermore, the quantized video data S15 is subjected to an inverse quantizing process and a inverted discrete cosine transformation process in the prediction encoding circuit 13 so as to be detected to the difference data. The pre-prediction frame data stored in a frame memory of the prediction encoding circuit 13 is modified by the thus decoded difference data so that a novel pre-prediction frame data is formed so as to be stored in the frame memory.
In addition, the prediction encoding circuit 13 calculates the correlation between the thus stored novel pre-prediction frame data and the transmission unit block data S11 so as to obtain movement detection data. If a determination is made that the degree of the movement is a small degree, block data of the pre-prediction frame data which corresponds to the above-described transmission unit block data S11 is read out so as to be supplied to the video data encoding circuit 12 as the predicted present frame data S12 together with the movement detection data. As a result, the difference data S13 which has been subjected to the process of encoding between frames can be obtained.
If the movement detection data was not detected (it means a fact that the degree of the change of the image of the transmission unit block which corresponds to the above-described transmission unit block data is an intense degree), the prediction encoding circuit 13 supplies predetermined reference data as the predicted present frame data S12 to the video data encoding circuit 12. As a result, the difference data S13, which has been subjected to the intraframe encoding process, is formed.
The video signal decoding device 4 receives the transmission data S17 transmitted from the video signal encoding apparatus 2 via the transmission passage 3 by its receiving buffer memory 22 via an interface circuit 21. Then, the transmission data S17 transmitted in each transmission unit is decoded to transmission unit block decoding data S18 by a variable length decoding circuit 23, an inverse quantizing circuit 24 and a inverted discrete cosine transformation circuit (DCT inverse transformation circuit) 25. Then, the decoding data S18 is converted into a video signal by a digital/analog conversion processing portion 26 so as to be transmitted as output video signal VD.sub.OUT.
In a case where the moving picture described with reference to FIG. 1 is transmitted in the structure shown in FIG. 2, the video data encoding circuit 12 is brought to the intraframe encoding mode when the image data of the image PC1 is supplied as the transmission unit block data S11 at point t=t.sub.1 shown in FIG. 1. As a result, it subjects the transmission unit block data S11 to the intraframe encoding process so as to transmit it as the difference data S13 to the discrete cosine transformation circuit 14. As a result, the transmission video data S16 is supplied to the transmission buffer memory 17 via the quantizing circuit 15 and the variable length encoding circuit 16.
Simultaneously, the quantized video data S15 obtained at the output terminal of the quantizing circuit 15 is subjected to the prediction encoding process in the prediction encoding circuit 13 so that the pre-prediction frame data denoting the transmission video data S16 transmitted to the transmission buffer memory 17 is stored in the frame memory provided for the prediction encoding circuit 13.
When the transmission unit block data S11 denoting the image PC2 at point t=t.sub.2 is then supplied to the video data encoding circuit 12, the prediction encoding circuit 13 supplies the predicted present frame data S12 obtained by adding movement vector data to the pre-prediction frame data to the video data encoding circuit 12 if the degree of the change of the image of the transmission unit block data S11, which has been newly supplied, is a small degree. As a result, the difference data S13, which has been subjected to the interframe encoding process, is transmitted from the video data encoding circuit 12.
If the degree of the change of the newly supplied transmission unit block data S11 is an intense degree, the prediction encoding circuit 13 supplies the predetermined reference data as the predicted present frame data S12 to the video data encoding circuit 12. As a result, the difference data S13, which has been subjected to the intraframe encoding process, is transmitted from the video data encoding circuit 12.
As a result, the video data encoding circuit 12 supplies the difference data S13, which has been subjected to the interframe encoding process or the intraframe encoding process, to the discrete cosine transformation circuit 14 at point t=t.sub.2. As a result, the difference data S13 denoting the change in the image between frames or within the frame is supplied to the transmission buffer memory 17 as the transmission video data S16.
The transmission buffer memory 17 receives the transmission video data S16 thus supplied before it successively transmits the stored transmission video data S16 as the transmission data S17 to the transmission passage 3 at a predetermined data transmitting speed which is determined by the transmitting capacity of the transmission passage 3.
Simultaneously, the transmission buffer memory 17 detects the residual data quantity so as to feed back residual quantity data S19, which is changed in accordance with the residual data quantity, to the quantizing circuit 15 and to the variable length encoder circuit 16. As a result, the transfer buffer memory 17 controls the quantizing step size in accordance with the residual data quantity S19 so as to adjust the quantity of data generated as the transmission video data S17. Consequently, a proper residual data quantity (a quantity which prevents overflow or underflow) can be maintained in the transmission buffer memory 17.
As described above, the difference data S13, which can be obtained after the input video signal VD.sub.IN has been converted into the transmission unit block data S11, is orthogonally converted into in the discrete cosine transformation circuit 14. Then, the transmission data S17 to be transmitted to the video signal decoding device 4 is generated in accordance with the transformation encoding data S14 which can be obtained due to the above-described orthogonal conversion. If the image of the transmission unit block data S11, which is desired to be transmitted at present, is intensely changed when the difference data S13 is obtained in the video data encoding circuit 12, the intraframe encoding process is performed. As a result, the data quantity to be transmitted is enlarged. On the contrary, if the image of the transmission unit block data S11 is not changed or if the same is a small degree, the interframe encoding process is performed. As a result, the quantity of data to be transmitted is reduced. Consequently, the quantity of data to be transmitted can be efficiently controlled so as to correspond to the change in the image of the input video signal VD.sub.IN.
When the encoding method which is adaptively selected in the video data encoding circuit 12 is used, it is necessary for the structure to be arranged in such a manner that, whenever the transmission unit block data S11 for each one unit block is transmitted, an identification flag denoting the method of encoding the data for the one block is inserted into the transformation encoding data 14 before the transformation encoding data S14 is transmitted from the discrete cosine transformation circuit 14 so as to be transmitted.
When the transmission unit block data S11 for one unit is converted into the transformation encoding data S14 in the discrete cosine transformation circuit 14 so as to be transmitted to the video signal decoding device 4, the video signal decoding device 4 must recognize the fact that the transmission data S17 which has been transmitted has been encoded by the interframe encoding process or by the intraframe encoding process when each of the transmission unit block data S11 is supplied. Thus, the video signal decoding device 4 is able to efficiently process the transmitted data S17.